The regulatory subunit of cAMP-dependent protein kinase (PKA) exists in two isoforms, RI and RII, which distinguish the PKA isozymes, type I (PKA-I) and type II (PKA-II). Evidence obtained from a variety of different experimental approaches has shown that the relative levels of type I and type II PKA in cells can play a major role in determining the balance between cell growth and differentiation. RI? transfected cells exhibit hyper-proliferative growth and RII? transfected cells revert to a relatively quiescent state. Profiling by microarray revealed equally profound changes in gene expression between RI?, RII?, and parental OVCAR cells.